Conductive core substrates used for printed circuits have dielectric layers which electrically insulates individual printed circuit conductors from a central conductive core. Among the advantages of such conductive core substrates are temperature equalization, ground plane shielding, dimensional stability, elimination of warp, and high structural strength. Conductive core substrates have been used wherein the conductive core is used to transfer heat out of an electronic package or as part of the electrical circuit, such as to carry ground voltage. The conductive core is also used to control in-plane expansion that is necessary for the surface mounting of leadless microelectronic packages.
The construction of conductive core substrates is rather difficult, especially when plated through holes (PTH) are required in order to connect both sides of the conductive core substrate electrically, such as in the case of a via-in-via conductive core substrate. A plated through hole is a through hole wherein the through hole wall has a coating or lining of conductive material. The conductive lining electrically bridges conductors on one side of the through hole with conductors on the other side the through hole. The PTH's must be electrically isolated from the conductive core to prevent shorting, which complicates manufacturing. Therefore, such conductive core substrates are not widely used and are now substituted by more expensive solutions.
FIG. 1 is a flow diagram illustrating one of the current methods used to fabricate a conductive core substrate. The method comprises providing a conductive core in the form of a thin sheet of conductive material 102. The conductive core is provided with one or more core through holes (CTH) 104, such as in a process using mechanical drilling, chemical etching, laser drilling, or punching. Dielectric material is laminated on both sides of the conductive core 106. The dielectric material consists of sheets of epoxy prepreg material, which, during the curing step, the epoxy resin flows to completely fill the CTH's forming dielectric plugs therein 108. A secondary mechanical process is used to provide a dielectric through hole (DTH) centered on the dielectric plug in the CTH 110. The DTH is smaller in diameter than the CTH, which leaves a layer of the dielectric material lining the CTH wall. Electroless copper (Cu) is deposited on the now dielectric-covered conductive core, including the DTH walls 112, followed by a heavier electroplating of Cu 114. This produces a plated through hole (PTH) that is electrically isolated from the conductive core by the layer of dielectric material lining the CTH. The electrical circuit is then fabricated on the conductive layer on each side by conventional processing.
The drilling of the conductive core and the dielectric plugs to produce CTH's and DTH's, respectively, is commonly done using a laser. Mechanical drilling is not suitable for producing through holes smaller than about 150 μm. Mechanical drilling is thus appropriate only for large-diameter through holes and larger pitches (spacing between through holes).
Current manufacturing yields for conductive core substrates are poor. Also, the costs associated with double drilling, and the problems associated with maintaining tolerances during the double drilling steps, are high. Therefore, conductive core substrates have not found wide acceptance in industry, but have been used for critical applications such as for temperature equalization in avionics where conventional cooling systems are too bulky and ineffective.
For example, to produce a PTH with a finished diameter of 1 mm, a 1.15 mm diameter DTH must be drilled in the dielectric plug to allow for copper plating to a minimum thickness of 0.05 mm on the DTH wall. Also, the dielectric layer on the CTH wall must have a minimum thickness of 0.25 mm to prevent the dielectric plug from being torn out of the CTH during the second drilling operation. In addition, allowance must be made for cumulative registration errors in the first and second drilling operations by adding another 0.08 to 0.13 mm to the diameter of the CTH. Thus, to produce a finished PTH with a diameter of 1 mm, the CTH must be drilled to a diameter of 1.73 mm to 1.78 mm. When CTH's are drilled on 2.54 mm centers, the double drilling operation leaves a web of core material between adjacent CTH's having a width of 0.76 mm to 0.81 mm. The CTH to CTH distance cannot be made smaller than 2.54 mm because the first CTH must be so much larger than the PTH. This makes it difficult to increase the component packaging density.
A method is needed to address the problem of the high cost associated with the secondary step of laser drilling of the dielectric plugs in the CTH's. This step is a time-consuming process requiring 15–20 laser pulses per CTH. This step accounts for an estimated 5–7% of the total substrate cost. The method needs to address the alignment issues associated with laser drilling the CTH's and DTH's. Alignment issues require that the CTH's and DTH's be sized larger and spaced further apart than one would need for a more accurate process. Further, issues of scalability and migration to finer PTH pitches, which are limited by current methods, needs to be addressed. Additionally, the method should be applicable to electrically conductive organic core material, such as resin/graphite based materials.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a significant need in the art for methods that address these issues.